Fish oil may help extend the lives of those patients, the heart association said.

On the other hand, the supplements cannot be recommended for preventing heart problems in the first place, said Dr. David Siscovick, lead author of the report.

That's because no clinical trials have tested fish oil's effects in people who are free of heart disease.
Meanwhile, a few trials have given fish oil to people at increased risk of heart disease -- because of diabetes or high cholesterol, for example. All but one has come up empty.

"Many people take fish oil hoping to prevent heart disease," Siscovick acknowledged. "But there's a lack of evidence that it does."

Fish oil is rich in omega-3 fatty acids, which are generally thought to be heart-healthy. Oily fish -- like salmon, tuna and mackerel -- are good sources, but many people turn to supplements.

One study found that roughly 8 percent of U.S. adults -- nearly 19 million people -- had used fish oil in the last month, according to the heart association.

The new recommendations are based on a review of 15 clinical trials that tested the effects of fish oil on heart health. Thirteen were conducted after 2002 -- the last time the heart association came out with recommendations on fish oil.

"We felt like the time was right to revisit this and review the evidence," Siscovick said. "We wanted to help guide doctors, because patients frequently have questions about fish oil."

Not everyone thinks the new recommendations are helpful, though.

"Frankly, I can't support this. I think it's a disservice to the public," said Dr. Steven Nissen, chairman of cardiovascular medicine at the Cleveland Clinic, in Ohio.

Nissen sees no convincing evidence that fish oil truly benefits heart patients. He noted that studies of heart attack survivors have come to mixed conclusions -- a point the heart association also makes in its report.

As for the heart failure recommendation, Nissen said, it's based on just one trial that showed a small effect on patients' outcomes.

"They [the heart association] should have said the jury is still out," he suggested.

Prescription-strength omega-3 is sometimes used to treat very high triglycerides. But people can, of course, get fish oil over the counter. That is part of what worries Nissen: Some heart patients, he said, might head to the health food store for fish oil -- possibly in place of their prescription medications.

"My fear is this will cause some serious public harm," Nissen said.

For its latest recommendations, the heart association analyzed clinical trials that focused on heart attack survivors. Each trial randomly assigned patients to either add fish oil to their standard medications or stay with standard care alone (and, in some studies, take placebo pills).

Two older trials (pre-2002) found that fish oil lowered patients' risk of dying or suffering a heart attack or stroke by about 15 percent. The more recent trials found no such benefits.

One 2008 trial tested fish oil in patients with heart failure -- a chronic, progressive disease in which the heart cannot pump blood efficiently enough to meet the body's needs.

Overall, patients on fish oil had an 8 percent lower risk of being hospitalized or dying over four years.

Based on those trials, doctors might want to add fish oil to their treatment arsenal, the heart association said.

But there are still questions: Why, for example, did the recent trials of heart attack patients show no benefits?

The heart association pointed to one potential explanation: Patients in the recent studies were much more likely to be on statins, or to have gotten angioplasty or another artery-clearing procedure after their heart attack.

So it's possible that fish oil has little effect when a patient is already on "maximal" standard treatment.

Why would fish oil benefit heart patients? According to Siscovick, the evidence suggests it's not by preventing artery-clogging plaques and warding off a heart attack.

Instead, he said, fish oil might help curb the risk of a life-threatening heart-rhythm disturbance.

The review looked only at studies of fish oil, and not omega-3 fatty acids from fish. But the American Heart Association has long recommended that people eat fatty fish at least twice a week.

The new report was published online March 13 in the journal Circulation.